Demodulating detector circuit arrangement



Feb. 25, 1941 [DLE 2,232,856

DEMODULATING DETECTOR CIRCUIT ARRANGEMENT Filed Feb. 24, 1959 INVENTOR aALLAN ELL/ IDLE ATTORNEY Patented F eb. 25, 1941 PAT NT OFFICEDEMODULATING DETECTOR CIRCUIT ARRANGEMENT Allan Ellis Idle, Chelmsford,England, assignor to Radio Corporation of America, a corporation ofDelaware Application February 24, 1939, Serial No. 258,127 In GreatBritain May 6, 1938 2 Claims.

This invention relates to demodulating detector circuit arrangements andmore particularly to such arrangements of the diode type. The mainobject of the invention is to provide simple demodulating detectorcircuit arrangements of the diode type which shall be of improvedfidelity as compared to the usual known arrangements of the type inquestion.

It is usual practice at the present time in radio receivers and the likehaving demodulating detectors of the diode type to couple the first lowfrequency amplifier valve to the preceding detector by means of acondenser-resistance network. This network has been found to be a sourceof distortion because, by reason of its presence, the A. C. and D. C.loads on the diode are not equal and also because it introducessubstantial damping into the detector.

According to this invention a demodulating decharacterised by theomission of the hitherto usual condenser-resistance output couplingnetwork, the detector output being taken direct from a resistive networkin which the detector device is included in series.

The invention is illustrated in the accompanying diagrammatic drawing.

Figs. 1-4 disclose different species of the invention.

Referring to Figure l which shows one embodiment of the invention verysuitable for use in a so-called straigh radio receiver where the inputcircuit is usually earthed on one side, modulated carrier wave energy tobe detected is fed from a source not shown to a coupling coil I (usuallytuned by a parallel condenser, not shown) which is connected at one endto the cathode 2 of a diode 3for example, as shown, a diode whosecathode is indirectly heated by a heater 4and is con- 40 nected at theother end through an output potentiometer resistance 5 to the anode 6 ofthe diode. Preferably a high frequency filter is included in the diodecircuit; for example, as

shown in Figure 1, a filter resistance 1 (or a 45 choke) may be insertedin series between the anode 6 and the adjacent end of the potentiometerresistance 5 and condensers 8, 9, connected between the coupling coilend of the potentiometer resistance 5 and. the two ends of the filterresistance or choke, as the case may be. Where the diode is alsorequired to provide uni-directional voltage for automatic volumecontrol, this voltage may be taken off as shown from the diode anode endof the potentiometer resistance 5 through a resistance II! (which shouldbe as high tector circuit arrangement of the diode type is as possibleto avoid increasing damping and distortion) to an AVC lead H, which maybe bypassed to the coupling coil end of the potentiometer resistance 5through a by-pass condenser 12. The valve l3 of the succeeding lowfrequency 5 stage has its input grid l4 connected to a tap I5 upon thepotentiometer resistance 5. This tap may be manually adjustable forvolume control purposes. The cathode [6 of the valve I3 is connectedtothe coupling coil end of the potentiometer resistance 5 through anauto-bias resistance I? (which may be of smaller value than at presentcustomary) through which a small negative bias may be applied tothe saidvalve grid. If this cathode resistance is not shunted by a by-passcondenser, it will introduce negative feed back and thus: exercise arectilinearising effect in the low frequency valve input circuit. Theanode circuit of the low frequency valve may contain in series with aresistance I8 a meter I9 and, for a given valve and an assumedmodulation percentage, this meter may be directly calibrated to readwatts output to the loudspeaker or other utilisation device (not shown)which is fed from the low frequency valve l3 through coupling condenser20. The meter scale may, if desired, be marked to indicate the maximumdesirable working limit. If no A. V. C. is provided the meter may beused as a tuning indicator. If desired the low frequency valve may bearranged as a phase changer valve with a load in its: anode and cathodecircuits feeding into a succeding pushpull or so-called paraphaseamplifier.

Although in Figure 1 the low frequency valve is fed from a simpleresistance (the potentiometer resistance) obviously it could equallywell be coupled to the detector by a resistance fed transformer.

A slight defect of the arrangement of Figure l is that thecathode-heater capacity of the diode is in parallel with the inputinductance coil I. In the modification shown in Figure 2 the diode isparallel fed from the coil I. With the circuit of Figure 2, which it isthought will be found self-explanatory from the figure, the detectordamping on the coil I is heavier than with the circuit of Figure 1. Aswill be seen, in Figure 2 the condenser 8 connects the live end of coill to the anode 6 of the diode.

The modification shown in Figure 3 is suitable for use in cases wherethe input coil I need not be earthed on one side, e. g. when it formspart of an intermediate frequency transformer in a superheterodynereceiver. In this figure the low potential side of the coil I isconnected to the 68 cathode 2 through resistance 1' in series withpotentiometer resistance 5', the tap IS on the latter being taken to thegrid M of valve l3. Resistance i in conjunction with the condensers 8"and 9' constitute a high frequency filter. The valve I3 feeds its outputthrough a push-pull output transformer which, with its associatedresistances 22 and condensers 23 forms afilter circuit designed inaccordance with well known principles to give a desiredfrequency-response curve. The transformer 2| feeds into a succeedingpushpull connected amplifier stage (not shown).

Figure 4 shows a further embodiment suitable for use for feeding (fromthe valve [3) a succeeding so-called paraphase amplifier stage. As willbe seen, in this figure two resistances I8 are providedone each side ofthe valve [3- and two coupling condensers If A. V. C. voltage isrequired an additional diode or diode anode must be provided. No A. V.C. circuit is shown in Figure 4. This circuit is remarkably free fromthe undesired frequency discrimination which has usually been present inknown cir cuits of the same general nature and which has arisen byreason of varying feed back voltages produced by the condenser usuallyprovided in such known circuits across the auto-bias resistance.

Obviously the push-pull and straigh output circuits of Figure 3 on theone hand and Figures 1 and 2 on the other, may be interchanged. In allthe figures the resistance I! may in some cases be omitted altogether asa diode will usually give from to 1 volt negative on its anode (withoutany signal input) due to electron bombardment from the cathode. Againthe valve I3 need not be a triode for other suitable valves, e. g.tetrodes or pentodes, may be used. A triode, however, is probablyslightly to be preferred for this Valve.

I claim:

1. A detector-amplifier system comprising in combination a dioderectifier having a cathode and an anode, a circuit including an inputcoil, a filter impedance and an output resistor all serially connectedbetween the rectifier cathode and anode, a condenser connecting each endof the filter impedance to the cathode, an audio amplifier having acontrol grid directly conduotively connected to an intermediate point onsaid output resistor, an unby-passed resistor connected to the cathodeof the amplifier for eflecting degeneration whereby to avoid undesiredfrequency discrimination in the output of the system, a connectionbetween the rectifier cathode and an intermediate point on the amplifierunby-passed resistor, and a pair of output terminals each connectedthrough a coupling condenser, one to the amplifier anode and the otherto the intermediate point on the unby-passed resistor.

2. A detector-amplifier system comprising in combination a dioderectifier having a cathode and an anode, means including a signal inputcircuit for applying modulated signals to said rectifier, an outputresistor connected in circuit with said rectifier device for derivingtherefrom alternating current signal potentials, an audio amplifierhaving a control grid directly conductively connected to an intermediatepoint on said output resistor, an unby-passed resistor connected tothecathode of the amplifier for effecting degeneration whereby to avoidundesired fre quency discrimination in the output of the system, aconnection between the rectifier cathode and. an intermediate point onthe amplifier unbypassed resistor, and a pair of output terminals eachconnected through a coupling condenser, one to the amplifier anode andthe other to the intermediate point on the unby-passed resistor.

ALLAN ELLIS IDLE.

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